1. Field of Invention
This invention relates to fluid treatment methods and apparatus for precipitation of metal ions, such as copper, nickel, cadmium, lead, zinc, and chromium, from fluids by chemical oxidation-reduction processes produced by electrolysis, commonly referred to as electrocoagulation (EC), and more particularly to electrocoagulation apparatus for dissolving replacement metals, such as iron and aluminum, by electrolysis to initiate the precipitation process and force the reactions to completion.
2. Description of Related Art
Organic compounds produced or used in industrial processes can contaminate the water used in the processes. Many industrial plants, such as petrochemical refineries and gas plants destroy the organic compounds in biological reactors before discharging the wastewater into public streams. Metals used in the processes, e.g. catalysts, and metals corroded from the stainless steel piping can also contaminate the wastewater. Wastewater contaminated with certain heavy metals cannot be introduced into the biological reactors because the metals kill the microorganisms that feed on the organic compounds. Thus, the metals must be precipitated and removed from the wastewater before the water is introduced into the biological reactors or discharged directly into public streams.
Governmental regulations restrict to very low levels the amount of contamination that can be discharged. Some of those low discharge levels dictated by regulation can be extremely difficult to reach using current systems. If the low limits of the regulations are exceeded, the fines for not complying with the discharge regulations can be substantial. Further, when the wastewater cannot be decontaminated in a timely fashion on the plant site to comply with the regulations, the water is hauled off to facilities specializing in wastewater processing or pumped down approved hazardous wastewater disposal wells. The regulations become even more restrictive if the hazardous wastewater is transported off the plant site where it is generated. The cost and accountability of transporting the wastewater off-site can become cost prohibitive. Some plants discharge hundreds and others even thousands of gallons per minute on a continuous 24-hours per day, 365-days per year basis. Similar wastewater issues are prevalent in other plants such as electric power plants or electroplating plants.
Electrical means have been used for some time to treat water and to reduce problems of encrustation or scaling due to mineral deposits. For instance, the present inventor invented such a system for treating water in an open re-circulating system as disclosed in U.S. Pat. No. 4,235,698, issued on Nov. 25, 1980. For that application, electrodes of the most inactive metals practical were selected for the apparatus on the basis of effectively treating the water only with the electrical signals without the electrodes going into solution.
Many attempts using various methods have been made to process wastewater on the sites where it is generated. Previous attempts to remove metals from wastewater have included electrocoagulation (a process where iron or aluminum plates, configured as sets of cascaded electrodes, are consumed by electrolysis as waste water passes over them) systems. It is known that typical electrocoagulation reactors employ electrodes of non-hazardous metals, such as iron (Fe+++) and aluminum (Al+++), that go into solution. Wastewater having hazardous metals is passed between the electrodes and a current is applied to the electrodes. The electrodes then form positive ions that can replace the ions of hazardous metals in compounds that keep them in-solution, so both the hazardous and non-hazardous metals can precipitate together (known as adsorption and co-precipitation). In this way, the metals are separated from the wastewater as solids.
The selection of the non-hazardous metals for the electrodes is based on several factors: availability and cost; their chemical activity relative to the hazardous metals to be removed indicated by their position in the chemical oxidation potential tables; the ease at which they can be ionized by electrical means in accordance with Faraday""s Law; the ability to concentrate them and increase the reaction potentials due to concentration in accordance with the Nernst equation; the ease at which they can be removed from solution following the removal of the other metals either by precipitation or plating out of solution; and the ability of the apparatus used to affect and contain the reactions.
The present electrocoagulation systems have been deficient in a number of ways. For instance, with present EC systems, the amount of metal contamination cannot be reduced to the regulated discharge limits. Further, present EC reactors require relatively large electrode surface areas. To provide the large surface areas needed from material generally available on the market, rectangular or square plates are used for the electrodes in many units. Those EC reactors are typically enclosed on all six sides by six exterior insulating plates of plastic.
These plastic plates bow and the units become visibly deformed when subjected to even moderate pressures on the inside of the plates. Because of this distortion, the plastic plates are difficult to seal. The larger the plates the more difficult it is to seal them. Many of the units may not withstand the pressures needed to operate at the stated flowrate. These units then leak the electrically-charged wastewater. Thus, those units are typically restricted to relatively small sizes with relatively small side panels and low flow rates.
Present electrocoagulation reactors generate large amounts of oxygen and hydrogen gases, by decomposition of water. In general in many industries, potential sources of electrical ignition are carefully protected by sealing them with explosion proof housings. However, the existing electrocoagulation units are typically not explosion proof. Thus, present EC reactors may leak leading to a potential safety hazard for the people immediately around the units.
Pure iron is not generally available for the electrode plates, so the iron needed for the EC reactor is obtained from steel plates. Steel electrodes have imperfections that cause the electrical erosion of the plate not to be uniform. As a result, small chips fall off the plates. The chips are conductive and can short the electrodes if they are lodged between the plates. Current EC systems are susceptible to this shorting problem.
For instance, U.S. Pat. No. 5,928,493 to Morkovsky et al. discloses mounting the plates horizontally. Large plates mounted horizontally sag as they erode to go into solution. The erosion is not uniform and the plates touch each other electrically shorting out the EC reactor. Morkovsky also discloses an EC reactor with electrode plates alternately unsupported at one end. Wastewater flows around the unsupported ends of the electrical plates in the apparatus disclosed as it zigzags through the reactor. The unsupported ends of the plates are exposed to the larger areas of adjacent plates during the treating action. Further, being exposed to a larger cathode area (from both sides) causes an anode to deteriorate faster. Faster erosion of the smaller area only causes the deficiency to become worse. This action has the adverse consequence of accelerating the sagging of the unsupported end of the plate and shorting the life of the entire plate assembly.
Further, the Morkovsky system utilizes costly clarifiers and a de-foam tank, both of which increase the costs of removing the heavy metals from wastewater. These limitations may also limit the capacity of the electrocoagulation system.
German Patent Application DE 3641365A1, published Aug. 25, 1988, discloses an apparatus for the cleaning and treatment of contaminated water using the electrocoagulation process. Again, this process utilizes the disadvantageous horizontal electrodes and clarifiers.
Manzione et. al describe using adsorption and co-precipitation to remove hazardous metals in discharges from power plants in xe2x80x9cField Evaluation of Arsenic and Selenium Removal by Iron Co-precipitation,xe2x80x9d Journal WPCF, Vol. 58, No. 1 (January 1986). However, the process described therein added iron as a liquid compound, such as ferric chloride, instead of using the electrolysis with iron cathodes and anodes. The Manzione process also utilizes costly clarifiers to hold the wastewater until the metals precipitated out. Although the amount of time the solution is held in the clarifiers may vary, holding the solution adds cost to the process, as do the clarifiers themselves.
As shown above, multiple attempts to develop improved devices and processes for removing impurities from wastewater continue to be made. For the foregoing reasons, there is a need for an apparatus and method to remove contaminants from wastewater in a timely fashion. It is desired to have an apparatus and method of removing hazardous metals from wastewater that does not add chloride ions, and have a reactor that can withstand operational pressures without leaking. Further, it is desirable that the improved process and apparatus does not require including costly clarifiers nor require the holding of the wastewater for excessive periods to time. The desired system would not utilize holding or defoaming tanksxe2x80x94items which increase costsxe2x80x94and would utilize explosion-proof components. It is also desirable to have the throughput or capacity of present systems to increase such that wastewater would not need to be hauled off-site. The desired system should be able to utilize iron electrode plates without being susceptible to the shorting caused by iron bits. Finally, it is desirable to have the wastewater treatment process to be performed in real time.
It will become clear to those skilled in the art having the benefit of this disclosure that the methods and apparatus in accordance with the present invention overcome, or at least minimize, the deficiencies of existing electrocoagulation apparatus and methods.
In some embodiments, the electrical fluid treating apparatus in accordance with the present invention removes metal contamination from aqueous fluids, such as wastewater, by precipitation in chemical oxidation-reduction reactions employing electrolysis to comply with government regulations for discharge into public streams. In some aspects, the present invention further enhances the fluid treatment by increasing the pressure under which the fluid is treated, providing an increase in dissolved oxygen and hydrogen gas exposure to the contaminants within the system during the treating process, and providing the pressure needed for separating the suspended solids from the fluid.
In other aspects, the electrical fluid treating apparatus of the present invention includes an electrocoagulation (EC) reactor, a direct current (DC) electrical power supply, a pressure vessel container for the EC reactor, a system pressure pump, valves and piping to direct the flow of fluid, and a monitoring and control system, and a cyclone filter.
The EC reactor of some embodiments of the present invention includes a support enclosure having multiple electrode plates disposed thereon. The electrode plates are insulated from each other; however, the electrodes remain in direct contact with the fluid as it flows between the electrodes. A DC power supply is provided to induce opposite charges on alternate electrodes thereby generating a strong electric field between adjacent electrodes to cause the electrodes to ionize and go into solution for interaction with the contaminants in the fluid as it flows through the apparatus. An embodiment of the fluid treating apparatus in accordance with the present invention includes parallel plate electrodes disposed on the support enclosure so as to be parallel with the direction of fluid flow between the electrode plates.
An alternate embodiment of the device in accordance with the present invention includes parallel electrode plates with apertures in the plates disposed on the support enclosure located so as to be in a traverse relationship to the fluid flow where the fluid flows through the apertures in the electrodes. The EC reactor of some embodiments of the present invention may be positioned in a pressurized container, where the exterior pressure on the EC reactor is higher than its internal pressure.
The DC electrical power supply of some embodiments of the present invention includes automatic adjustment of the voltage to provide a constant preset current and automatically reverses the direction of current at adjustable preset intervals. The DC electrical power supply can be housed on an explosion proof housing and can be operated by a remote control and monitoring system.
The container of the present invention includes a horizontal flange around the diameter to divide the vessel into a lower section and a removal upper section that serves as a cover. The lower section of the container includes a support structure on which the EC reactor is positioned. An embodiment of the container is classified as a pressure vessel in accordance with the ASME Pressure Vessel Code Section X Class I or Class II capable of operating at various pressure ranges, such as from 10 to 100 psig. The pressure vessel of some embodiments is made from high temperature fiber reinforced plastic capable of resisting a variety of acids, bases, and solvents. The plastic material does not conduct electricity. A number of flange connections are provided in the pressure vessel to allow fluid to flow in and out as it is being treated, to drain the vessel, to vent gases during operation, allow air to enter the vessel during draining, and to supply electrical power.
In some aspects, the system pump of the present invention supplies the fluid to be treated at the pressure and flow rate the EC system is to be operated.
The valves and piping of some embodiments of the present invention direct the flow of fluid through the EC system. The valves are operated by the monitoring and control system. Valves and piping are made of materials that do not conduct electricity.
The monitoring and control system of some aspects of the present invention includes a programmable logic controller; sensors mounted in the outlet piping for flow, temperature, pressure, and pH; and interconnecting wiring. The controller can be mounted in an explosion proof housing. In some embodiments, a cyclone filter is utilized to separate solid contaminants from clean wastewater.